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The correct answer is C) Acute promyelocytic leukemia. This was a challenging case and differential, but the presentation and smear were classic and are exactly what you would be expected to know on a USMLE-style exam.
APL is a distinct subtype of AML (AML-M3 in the French-American-British classification system). There are approximately 600-800 cases diagnosed in the US yearly. Unlike other forms of AML, the incidence of APL peaks in early adulthood.
The presentation of APL is typically related to complications from pancytopenia. Fatigue, bruising, fevers/infection, and epistaxis (nosebleed) are common complaints. Presentation with purpura or disseminated intravascular coagulation (DIC) is a classic presentation that is unique to APL (versus other leukemias).
Suspected APL is a medical emergency and warrants urgent admission for management of complications and empiric treatment with a differentiation agent, typically all-trans retinoic acid (ATRA). The pathogenesis of APL is dependent on a t(15;17) translocation which results in arrested development of neutrophil precursors and subsequent accumulation in the bone marrow (see below).
ATRA can be safely administered while cytogenetics are pending, and is easily discontinued if an alternate diagnosis is made. In this case, characteristic Auer rods were observed on the bone marrow aspirate; in particular, a “faggot cell” (from the English expression for a bundle of sticks) is observed (see below), which is virtually pathognomonic for APL.
The patient’s APL was confirmed by cytogenetics and he was initiated on a regimen of ATRA and arsenic trioxide. Cure rates for low-intermediate risk APL such as this case have cure rates >90% with timely intervention.
While hemophagocytic lymphohistiocytosis (A) does cause a pancytopenia, it is not the most likely cause of this patient’s presentation. Hemophagocytic lymphohistiocytosis (HLH) is a rare hematologic condition that typically presents with fevers, splenomegaly, and evidence of hemophagocytosis on microscopic examination of bone marrow/blood smears.
Although viral infection (B) can result in pancytopenia, this is not likely in the presented case. Pancytopenia is most commonly associated with HIV, but is also seen with Epstein-Barr Virus infections and viral hepatitis. This patient had a negative HIV test and no risk factors, signs, or symptoms of EBV or hepatitis infections.
Severe vitamin deficiencies (D) can cause pancytopenia, but this patient lacks risk factors, did not have any physical findings associated with relevant vitamin deficiencies (e.g. atrophic glossitis in B12 deficiency). One might also expect characteristic findings on peripheral smear, including macrocytosis and hypersegmented neutrophils.
Systemic lupus erythematosus (E) is commonly associated with cytopenias, but is very unlikely to be the cause of pancytopenia in a young male without other signs or symptoms of SLE.
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